For example, brush attached electric motors are usually used as a wiper motor of an automobile. In this type of electric motors, a plurality of permanent magnets are arranged on an inner circumferential surface of a yoke having a cylindrical shape at even intervals in a circumferential direction, and an armature is rotatably supported on an inner side of the permanent magnets. For the permanent magnets, a so-called segment-type permanent magnet that is formed in a substantially tegular shape (substantially arc shape) is used.
The armature has an armature core in which a plurality of teeth are radially formed. A plurality of slots that are elongated in an axial direction are formed between the teeth, and a winding wire is wound around the teeth via the slot. The winding wire is electrically connected to a commutator that is externally fitted and fixed to a rotation shaft so as to be adjacent to the armature core.
In the commutator, a plurality of segments which are metal pieces are arranged in the circumferential direction in a state of being insulated from each other, and a wind starting end and a wind finishing end of the winding wire are connected to each of the segments. Each of the segments is connected slidably to the brush, and electric power is supplied to each winding wire via the brush. Then, a magnetic field is formed at the teeth by the winding wire that is supplied with the electric power, and the armature is rotated by a magnetic suction force or repulsion force that occurs between the magnetic field and the permanent magnet provided on the yoke.
A variety of techniques are disclosed in order to reduce factors according to motor operation sounds such as a cogging torque and a torque ripple.
For example, a technique is disclosed in which the position of the arc center of the inner surface of a permanent magnet having a tegular shape is decentered with respect to the position of the arc center of the outer surface to prevent an abrupt magnetic flux change in the circumferential direction on the yoke side (for example, refer to Patent Document 1).
Further, a technique is disclosed in which a process is applied on an opposed surface of a permanent magnet of teeth to reduce a density change in a magnetic flux generated at the teeth (for example, refer to Patent Document 2).
Further, a technique is disclosed in which a polar arc angle of a permanent magnet is limited to thereby reduce a cogging torque (for example, refer to Patent Document 3).
Further, a technique is disclosed in which teeth or slots are formed to have a skew angle so as to diagonally extend with respect to an axial direction (for example, refer to Patent Document 4).
According to such a configuration, it is possible to shift, in the axial direction, occurrence timings of a magnetic suction force or repulsion force that occurs between a magnetic field by a winding wire and a permanent magnet provided on a yoke.
When teeth or slots have a skew angle, it is possible to derive an optimum skew angle from the lowest common multiple between the number of magnetic poles and the number of slots. That is, a value obtained by dividing 360° by the lowest common multiple between the number of magnetic poles and the number of slots is an optimum skew angle. For example, when the number of magnetic poles is 4 and the number of slots is 6, the lowest common multiple is 12, and the value obtained by dividing 360° by 12 is 30°. Accordingly, when the number of magnetic poles is 4 and the number of slots is 6, the optimum skew angle is 30°.